wright



Feb. 2, 1960 Original Filed Aug. 2, 1952 A. M. WRIGHT FLUID FLOW CONTROLS 2 Sheets-Sheet 2 2 mm (g ATTORNEY United States Patent FLUID FLOW CONTROLS Alexander M. Wright, West Hartford, Conn.

Original application August 2, 1952, Serial No. 302,441,

now Patent No. 2,788,063, dated April 9, 1957. Divided and this application June 12, 1956, Serial No. 590,975

7 Claims. (Cl. 340-236) This invention pertains to fluid flow control apparatus, and more particularly has reference to fluid pressure sensing devices of the type disclosed in my copending application, Serial No. 302,441, filed August 2, 1952, of which this application is a division.

In aircraft propulsion, and especially for high speed, turbojet aircraft, it is highly desirable to detect an incipient failure of the engine fuel supply system during take-01f, so that where an emergency control is provided, an automatic switchover to emergency may be made without delay.

One solution of this problem is to provide a sensing device that will operate on rate of change of fuel injection nozzle pressure, so that for a slow drop in nozzle pressure, such as occurs during a normal climb, no action will result, but a response will be obtained from a more rapid drop in pressure, such as would occur due to a failure of the normal fuel supply system.

Accordingly, the principal object of this invention is to devise an emergency sensing device that will detect an abnormal falling off of fuel nozzle pressure in the fuel supply system of a turbojet engine during take-off, when a failure of the fuel system is most dangerous and requires immediate correction in order to avoid disaster.

Another object of this invention is to provide in such a device means for automatically switching the fuel supply to the emergency system and, at the same time, indicating to the pilot that such change-over has occurred.

Still another object is to devise an emergency sensing device having means for pre-fiight checking to insure that it is in proper working order before commencing flight.

A further object is to provide an emergency sensing device that will operate automatically whenever the rate of fuel nozzle pressure drop exceeds a selected value.

With these and other objects in view which may be incident to my improvements, my invention consists in the combination and arrangement of elements hereinafter disclosed and illustrated in the accompanying drawings in which:

Figure 1 shows the pertinent parts of an aircraft engine fuel control apparatus, such as is disclosed in copending application, Serial No. 302,441, mentioned above, in which is incorporated an emergency sensing device according to my invention;

Figure 2 is a central vertical section (partly diagrammatic) of one embodiment of my invention;

Figure 3 is a diagram of the electrical circuits connecting the emergency sensing device shown in Figure 2 with other elements of the fuel control apparatus shown in Figure 1; and

Figure 4 is a diagram showing the characteristic response of my emergency senser to a steady rate of drop in fuel nozzle pressure.

Referring to Figure 1, the reference numeral 1, denotes a fuel control apparatus of the type disclosed in copending application, Serial No. 302,441, mentioned above, which receives fuel from a supply tank 2 through a conduit 3, a fuel pump 4 driven by the engine and a conduit 5. A return conduit 6 provides for the return to conduit 3 of such part of the fuel delivered by pump 4 as is not required by the engine. Control apparatus 1 delivers fuel through a conduit 7 to a series of burner nozzles 8 in the combustion chamber 9 of a turbojet engine, in accordance with the requirements of the engine under various operating conditions.

Broadly comprehended, fuel control apparatus 1 comprises a main (normal) fuel supply system, in which fuel flows from inlet conduit 5, through a switch-over valve 10, conduit 11, check valve 12, conduit 13, automatic flow regulating valve 14, conduit 15, and chamber 16, to outlet conduit 7; and an emergency fuel supply system, in which fuel fiows from inlet conduit 5, through switch-over valve 10, a conduit 17, manual flow regulating valve 18 and chamber 16, to outlet conduit 7.

In the main (normal) fuel supply system, such part of the fuel delivered by pump 4, through inlet conduit 5, as is not required by the engine, is returned to the inlet side of the pump, through a conduit 19, pressure regulating valve 20, conduit 21, and conduit 6; while that portion of the fuel which is delivered to the burner nozzles 8 (through check valve 12, conduit 13, valve 14, conduit 15, chamber 16 and conduit 7) is regulated by'the conjoint operation of pressure regulating valve 20' and automatic flow regulating valve 14, both of which are controlled by other parts of fuel control apparatus 1 (not shown). In the emergency fuel supply system, such part of the fuel delivered by pump 4, through conduit 5, as is not required by the engine, is returned to the inlet side of the pump through a conduit 22, pressure regulating valve 23, conduit 24 and conduit 6; while that portion of the fuel which is delivered to burner nozzles 8 (through conduit 17, valve 18, chamber 16, and conduit 7) is regulated by the conjoint operation of pressure regulating valve 23 and manual flow regulating valve 18. Pressure regulating valve 23 is controlled by other parts of fuel control apparatus 1 (not shown), while manual flow regulating valve 18 is operated manually by the pilot. Valve 18 has a contoured surface whereby its flow area is progressively increased as said valve is raised from its lowest (closed) position by operation of the pilots manual control lever (not shown). Integral with valve 18 is a fuel cut-oft valve 18a (in the form of a disk) which is adapted to seat, with a fluid-tight fit, in a recess in the lower end of chamber 16 when valve 18 is in its lowest operating position, whereby it cuts off all fuel flow to the engine.

Switch-over valve 10 comprises a double headed valve, of which the head 25 is integral with a stem 26 on which is adjustably mounted a head 27 that is biased toward its seat 28 by a spring 29. Valve head 25 has an integral sleeve 30, slidably mounted in a cylinder 31, and is biased toward closed position by a spring 32. Cylinder 31 is connected through a conduit 33, solenoid valve 34, and conduits 35 and 24, to return conduit 6. When the solenoid 36 which actuates valve 34 is energized (as hereinafter described), valve 34 is raised to its upper position (as shown in Figure l), in which it cuts olf communication between conduits 33 and 35; and when solenoid 36 is deenergized, it is moved to its lower position by a spring 38 and establishes communication between conduits 33 and 35. I

opens said valve (as shown in Figure 1).

During operation under normal conditions, solenoid 36 is energized and cuts off communication between conduits 33 and 35, This blocks the escape from chamber 31 of fuel which entered therein through a restriction 37 in valve head 25, and thus permits the fuel pressure on both sides of valve 25 to equalize, so that spring 32 maintains valve 25 in closed position, while the fuel pressure, acting on valve head 27, overcomes spring29 and So long as valve 25 remains closed and valve 27 opens, the fuel supply-flows through the main (normal) fuel supply system (i.e,, through 11, 12, 13, '14, 15 -and16), and there is no flow through the emergency fuel supply system 17, 18 and 16).

Upon a failure of the main (normal) fuel'supply system, solenoid 36 becomes de-energized (as explained hereinbelow), whereupon valve 34 moves down and establishes communication between conduits 33 and 35.

This permits fuel to escape from chamber 31, through conduits 33, 35, 24 and 6, faster thanitcan enterchamber 31 through restriction 37, so that the pressure to the left valve 25 overcomes spring 32, opens valve 25 and P closes valve 27. The fuel supply now flows through the emergency fuel supply system (17, 18, 16 and 7), to

burner nozzzles 8-and there is no fuel flow through the main (normal) fuel supplyisystem.

7 When the main (normal) fuel supply system is in operation, the control pressure, which governs the operation of pressure regulating valve 20, issupplied from the control apparatus (notshown) through conduit 120, and is always less than the discharge pressure of pump 4,

as measured by the force of the spring biasing valve '20 toward closed position. Hence, valve 20 regulates the flow of by-passed fuel through conduits 21 and 6 around pump 4. Solong as the 'main (normal) fuel supply ,sys-

tem is operating, control pressure ,from conduit 120 is and 122, and since said control pressure exceeds the pressure'of the by-passedfuel in conduits 2,1 and 24-, the

transmitted to the bottom of valve '23 by conduits 12,1

same time, opens communication between conduit 122 V and a conduit 123, which supplies the emergency control pressure from the control apparatus (not shown), whereupon the valve 23 is operated by the emergency control pressure from conduit 123.

A fuel nozzle pressure sensing-device, embodying my invention and denoted by the reference numeral '40 in Figure 1, is connected by a conduit 40a to fuel outlet conduit 7. This device, as shown in detail in Figure 2, comprises a housing of three parts 41, 42 and 43 which are securely held in assembled position as shown by-any suitable means, such as connecting bolts (not shown). Housing parts 41 and 42 define a gas pressure chamber 44, in which is mounted a flexible (rubber) membrane 45 whose outer edge is securely clamped between parts 41 and 42 so that chamber 44 is air-tight. In its most distended position, membrane .45 conforms to the inner contour of housing part 42 so as to inclose aspace (44) of selected, fixed volume (v Housingpart 41 is provided with a threaded socket 46 for the reception of a threaded plug 47 whose inner end seats against the bottom of socket 46 and whose outer end has an hexagonal shape for the application of a wrench (not shown). Chamber 44 is connected to socket 46 by a port 48 and plug 47 has a threaded bore 49 which may be closed by a threaded pin50. The lower end of plug 47 is provided with a transverse bore 51 which communicates with bore 49. When pin 50 is removed and plug 47 is unscrewed so as to disengage its lower end from sealing contact with socket 46, a gas charging conduit (nq sshown) may be screwed into threaded bore 49 and chamber 44f may upon, the volume of said gas is the same as the selected volume (v of chamber 44. Plug 47 is then tightened so as to seal socket 46, the charging conduit is removed, and bore 49 is closed by inserting pin 50.

Housing parts 42 and 43 inclose fuel nozzzle pressure chambers 52 and 53, which are separated by a flexible diaphragm 54 whose outer :edge is securely clamped between parts 42 and 43, so as to form an air-tight joint. Housing part 42 is centrally bored for the reception of a flanged bushing 55 which is secured to diaphragm 54 by a threaded stud 56 having a flanged head 57. The flange 58 of bushing 55 fits slidably into a countersunk recess 59, in the wall of housing part 42; the depth of said recess being somewhat greater than the thickness of flange 58, so that said flange may projectvbushing 55 to the right into chamber 53, to the extent of said excess depth before contacting the bottom of said recess.

A plurality of passages 42a, in the sidewall of housing 42, permits the liquid pressure in chamber 52 to act to the left on membrane 45, and compress the gas in chamber 44 until its pressure rises from its initial charging pressure (p to .a higher pressure (p equal to the liquid pressure in chamber 52. This compression of said gas reduces its volume from its initial volume (v toa smaller volume (v in proportion to'the ratio (p /p on membrane 45, exceeds the initial charging pressure (p in chamber ,44, membrane 45 will be moved to the left ,-out of contact with the wall of part 42 carrying the passages 42a, until the pressure '(p' in chamber 44 rises (bycompression of the gas therein) to a value equal to the pressure in chamber 52; and as long as membrane 45 is out of contact with said wall, the pressure in chamber 52 will always equal the pressure in chamber 44, by virtue of the free movement of membrane 45.

Since membrane 45 can be moved to the left, out of contact with its supporting wall, only where the pressure (17 in chamber 52 exceeds the selected initial pressure (17 in chamber 44, saidpressures will remain equal in value only as long as the pressure in chamber 52 exceeds the selected pressure (p If now thepressure in chamber 52 decreases to a value equal to the selected value (12 -i.e., psi, membrane 45 will move to the right, until it contacts its supporting wall (as shown in Figure 2), whereupon the pressure (p in chamber 44 (and hence in chamber 52) will be equal to the selected charging pressure (p Any further decrease in the pressure in chamber 52 below (p =l00 p-.s.i.), will not appreciably reduce the pressure in chamber 44 below the value of the selected charging pressure (p because membrane 45, being now confined by the boundary walls of the diaphragm chamber, cannot move further to the right to increase the volume of gas in chamber 44 be yond the selected volume (v and any "addition to vol- :ume (v from the very slight movement of flange 58 to burner nozzles '8. Chamber 53 is connected to bore 61, between restrictions 62 and 63, by a passage 65. ,Restricition 63 is larger than restriction 62, so that when the ,fuel pressure (p ,in inlet 64 falls below the fuel pressure (p in chamber 52, fuel escapes from chamberu5i through passage 65 and restriction 63, faster than the fuel in chamber 52 can escape through restriction 62. A pressure differential (p -p is thus set up which acts on diaphragm 54 and moves said diaphragm (together with bushing 56) to the right until flange 58 of bushing 55 contacts the bottom of recess 59.

Slidably mounted in a transverse partition 66 in chamber 53 is a stud 67, having atits left end a rounded head 68, and at its right end, a plate 69 which is adapted to contact the ends of a pair of hooked pins 70 and 71 mounted in the right end wall of chamber 53, and electrically insulated therefrom by insulations 72 and 73. A spring 74 biases plate 69 and stud 67 to the left against the force of diaphragm 54. When the fuel pressure (p in chamber 53 is equal to the pressure (p in chamber 52 and there is no force exerted by diaphragm 54, spring 74 maintains plate 69 in contact with pins 70 and 71, but when the fuel pressure (p in chamber 53 falls below the pressure 1 in chamber 52 so that the pressure differential (p -p acting on diaphragm 54 exceeds the force of spring 74, stud 67 and plate 69 are moved to the right and break contact between plate 69 and pins 70 and 71. The total movement of stud 67 and plate 69 is only a few thousandths of an inch, so that plate 69 functions as a microswitch across pins 70 and 71. A plug 75 threaded in the right end wall of ghamber 53 permits adjustment of the tension in spring The electrical circuits which connect the fuel nozzle pressure sensor, just described, with other elements of the fuel control system of Figure l, are shown in Figure 3, wherein the pin 70 of Figure 2 is connected by a wire 76, through a main throw switch 77 to the positive pole of a battery 78. Wires 79 and 80 connect pins 70 and 71, respectively, with contacts 81 and. 82 of a three-position switch 83, whose third contact 84 is connected by a wire 85 with one contact 86 of a test switch 87. A spring 88 biases switch 87 in a normally closed position against contact 86 and a second contact 89, but switch 88 may be opened by pressing push button 90 to the left, so as to overcome the force of spring 88; for the purpose of testing the electrical system, as hereinafter described.

Contact 89 is connected by wires 91 and 92 to one contact 93 of a reset switch 94 which is normally held in open position by a spring 95, but which can be closed momentarily by pressing push .button 96 to the right,.

against the force of spring 95, until connection is made between contact'93 and a second contact 97. Contact 89 is also connected by wires 91 and 98 to one contact 99 of a solenoid relay lockout switch 100 which is held in open position by a spring 100', when the solenoid 102 is de-energized. However, when solenoid 102 is energizcd, it moves switch 100 to closed position against contact 99, and a second contact 101.

Contacts 97 and 101 are connected by wires 103, 104 and 105 with a junction 106 of a divided circuit whose other junction point 107 is connected by a wire 108 to the negative pole of battery 78. Junctions point 106 and 107 are connected: by wires 109 and 110 through an electric signal lamp 111 on the control panel in the pilots cockpit; by wires 112and 113 through solenoid 36 of valve 34 (Figure 1); and by wires 114 and 115 through solenoid 102.

Operation square inch (p.s.i.), and is then sealed, as described hereinabove, The flexible membrane 45 will then contact the right wall of chamber 44, as shown in Figure 2, and the volume of the gas in said chamber will have a selected value'(v Fuel at nozzle pressure (11 will flow from "conduit 7 through conduit 40a, inlet64and restriction 63 into bore 61 and from thence, through passage 65, into chamber 53 and also, through restriction 62 and passage 60, into chamber 52. After a time the pressure in chamber 52 will equal the (p in chamber 44 because of the passages 42-a between said chambers and the flexible membrane 45. The pressure in chamber 52 will also be the same as the pressure (p in chamber 53. In this state, since the pressure on both sides of diaphragm 54 is the same, no force will be exerted by said diaphragm upon stud 67 and spring 74 will hold switch 69 in closed position.

If now the pressure (11 in inlet 64 decreases, a flow of liquid will occur simultaneously from chamber 53 through restriction 63 and from chamber 52 through the much smaller restriction 62, whose effective area is (A,). Since the liquid escapes from chamber 53 much faster than it can escape from chamber 52, there appears across restriction 62 and diaphragm 54 a pressure drop (p p which tends to force said diaphragm to the right, against the resistance of spring 74. If the pressure drop (p p is large enough to overcome the force of spring 74, the switch 69 will open. If the drop in (p;.;) pressure is very slow, the differential (p p will be too small to open switch 69. The response of the system to different rates of reduction of nozzle pressure (12 is shown by the curves in Figure 4.

Starting with the engine at rest and the electrical system (Figure 3) de-energized, the battery switch 77 is closed to energize the electrical circuits in Figure 3. Assuming the emergency switch 83 has been left in nor- 'mal (position A in Figure 3), closing the reset switch 94 energizes the emergency solenoid 36, simultaneously lighting the indicator lamp 111 to indicate to the pilot that the fuel control apparatus 1 is on the main (normal) fuel supply system. At this time, the lockout relay solenoid 102 is also energized to close switch 100 and complete the circuit through the rate of pressure" or emergency senser switch 69.

After the engine has been started and brought up to speed, an emergency check may be made as follows:

(a) Place emergency manual switch at take-off (position B, Figure 3).

(b) Press button to open test switch 87. This throws the fuel control apparatus, explained in column 3 hereinabove, onto the emergency fuel supply system by de-energizing the emergency solenoid 36, at the same time de-cnergizing lockout relay coil 102, which opens the lockout relay switch 100, and prevents the fuel control apparatus from returning to main fuel supply system, when the test button 90 is released. At this time, the control indicating light 111 goes out, showing that the fuel control apparatus is on the emergency system. The actual transition from main to emergency can usually be felt as a momentary drop in propulsive thrust.

(c) Press button 96 to momentarily close reset switch 94, which restores the fuel control apparatus to the main fuel supply system.

(d) Place emergency manual switch 83' at normal (position A, Figure 3).

When the engine has been brought up to the required speed (r.p.m.), and prior to starting the take-off run, the manual emergency switch 83 should be placed at takeoff (position B, Figure 3). This arms the system for automatic switch to emergency operation, in case of an abnormal falling off of fuel nozzle pressure (12 After the airplane has taken off and climbed to a safe altitude, and before throttling back to cruising speed, the manual emergency switch 83 should be placed in normal (position A, Figure 3). If this is not done, throttling back the engine may throw the control apparatus into emergency operation.

In case of an automatic switch to emergency during take-off, the lockout relay solenoid 102 becomes deenergized. If this feature were not provided, the rcestablishment of nozzle pressure (12 after theslwitchover of the control apparatus to emergency operation would cause the fuel control apparatus to return to fmain fuel system operation, and there would be continuous .oscillations between the emergency and main systems, which condition is avoided by locking out .the -rnain system.

During flight, the control apparatus may be "placed in emergency operation'by throwing the manual switch 83 from normal (position A) to emergency (position C).

In the test procedure outlined above, the actual functioning of the nozzle pressure sensing switch 69 is not verified, and as an alternative scheme, it may be desirable to have vthe test switch operate a fuel dump valve, so that an actual loss of nozzle pressure can be simulated. Such an actual loss of nozzle pressure can also be obtained by pulling back on the throttle, and with the manual switch 83 at take-ofi (position B), this will switch the fuel control .apparatus -to femergency operation.

'The following analysis shows the principles of operation of the rate of pressure drop sensing device shown in Figure 2, in terms of the following nomenclature:

A =Eifective area of diaphragm 54, sq. in.

A =Flow area of restricting passage 61, sq. in.

,C=Coefficient of flow through restricting passage 61, cu.

in. per p.s.i.

er Exponential, 2.71828 k ==Spring rate of spring 74 #/inch p =Initial charged pressure of gas in chamber 44, p.s.i.

p. =Control chamber (44) pressure, p.s.i. p =Nozzle fuel pressure, p.s.i.

q=Flow through restriction 61, in. per sec.

-t=-Time, seconds 'v=Volume of liquid in chamber 52 (combined volume of the liquid in chamber 52 and in chamber 44, to the right of membrane 45) in.

v =lnitial charged volume of gas in chamber 44 v =Volume of gas in chamber 44, infi, under pressure x=Travel of diaphragm 54 (Le, plate 57) r=Time constant, seconds Equilibrium of diaphragm 54 is described by:

where x is the'length of the spring 74, x being its free length, and -(x x')=x, is the travel of diaphragm 54.

Now suppose, starting from a condition of p =p that the pressurein chamber 53 starts to fall at a constant rate 'During this fall in pressure, p p Then at any time t,

the pressures will be p and p and the equilibrium is described by (1).

'Flow through restriction 62:

q=.C r(p'l-Pn) The flow (q) through 62 is caused by and equal-to the expansion of the air'behind diaphragm 45, minus the volume absorbed by the movement of diaphragm 54 to the right (in the positive x direction), hence also iflerenti n With isothermalconditions in chamber 44, wezhave:

FFIYFPDVG um-p1) ,8 hence 1 .and

i j1 1 0 0 2 1 I dt" 1?? d:

,Substituting (3) and (3a) in (.2), and then equating (2)-and (2a):

To .predict the performance of the device, we assume "that 1- is constant during its action, whichis fairly ,close to reality since the action takes place duringa smalldisplacement of the diaphragm-54.

Then the solutionof (7) is- Let it be assumed that the device shown in Figure-'2 is to detect a pressure rate of change of 20:p.s.i.-per Second, and specify that this is to'provide a (p -p of 2 p.s.i. for actuating thevdiaphragm 54 when p =600 psi.

To keep the charging pressure p from getting too high, CA, should be made asilow vas possible, say .01.,cubic inch persecond per p.sLi.

For example, if we arbitrarily choose 7:.25 second.

Then in (9) put 5%?20 p.s.i./ sec. CA,.=.01 cubic inch/sec. per p.s.i. p =600 p.s.i. "r=-2 5 Equation -9 then is:

2:20 ttl l -3fi t lf 00 X {1 36 po oX 1. 3.

Af er --a o g ti he expone rm disappears, and

p v =360 I The design of chamber. 44 and membrane 45and'the -.chat iasm essunc.mus thensatisfy thi lastemdttmn- If the charging pressure is 100 p.s.i., then the volume of gas in chamber 44 must be 3.60cubic inches when p,

In this example, if the chamber size is 3.60 cubic inches,

then the volume of gas therein can never exceed this, and the device will not hold the emergency switch 69 open when p, falls below 100 p.s.i., absolute, until 11 falls below 96 p.s.i., for the following reasons. The volume of fuel per second (q) flowing out through restriction 62 is determined by the volume of fuel in the chamber between membrane 45 and the wall carrying passages 42a, which in turn is equal to the difference between the maximum volume (v of gas in chamber 44 (when p =p =100 p.s.i.), and the reduced volume (v;) in chamber 44, when the fuel pressure (p in chamber 52 exceeds the charging pressure (p in chamber 44. As long as the pressure differential (p -p resulting from the fuel flow (q) through restriction 61, exceeds that required to overcome the force of spring 74 (e.g., 4 p.s.i.) valve 69 will open and remain open. However, when the pressure (12 in chamber 44 (owing to the discharge through restriction 62), falls to a value of the assumed charging pressure, viz., 100 p.s.i. (i.e., p =p membrane 45 is in contact with its supporting wall carrying passages 42a, and liquid flow through restriction 62 ceases, whereupon the pressure differential (p -p becomes zero, which is less than that (e.g., 4 p.s.i.) required to hold valve 60 open, and hence when (p falls to a value of 100 p.s.i.=(p or below, the device will not hold switch 69 open, until p falls below 96 p.s.i.

Putting the above constants in Equation 10, and op eration of the unit, we get i Equation 11 shows the response of the unit to the time rate of change of the pressure (pm) in chamber 53, for these conditions; as indicated by curve C of Figure 4, wherein the ordinates show the unit differential pressure (p -p acting on diaphragm 54, and the abscissas show elapsed time of response, in seconds.

Thus, if the unit differential pressure (p -p is changing at the rate of 100 p.s.i. per second, the unit will respond as shown in curve A of Figure 4; if said rate of change is 50 p.s.i. per second, the response is as shown by curve B; while if said rate of change is 20 p.s.i. per second (as assumed in the foregoing example resulting in equation 11), the response is as shown in curve C. If a unit differential pressure (p p of 4 p.s.i. is required toopen switch 69, it is apparent from curves A, 13" and C of Figure 4, that switch 69 will open in .08 second for a rate of change of (p p of 100 p.s.i. per second; and in .21 second, for a rate of change of (p -p of 50 p.s.i. per second; while said switch will not open at all when the rate of change p P falls to 20 p.s.i. per second, until 12,, falls below a value of 96 p.s.i.

When p in chambers 53 and 52 falls below p =l p.s.i., a differential pressure of (1OO-p acts on bushing 55 (over the area of flange 58), and this pressure differential continues to increase as p continues to fall further below 100 p.s.i. When p falls to a value such that the pressure differential 100-12 applied to the area of flange 58, produces a force (acting to the right on bushing 55) greater than that of spring 74, switch 69 will open. Since the area of flange 58 is smaller than the area of diaphragm 54, the pressure differential (100-p-), acting on flange 58, must exceed 4 p.s.i., (which, when acting on said diaphragm 54, is just sufficient to open switch 69), by an amount equal to the difierence in areas of flange 58 and diaphragm 54. If these two areas were equal, switch 69 would open whenmasses ever p was less than 96 p.s.i., i.e., (p p-)=4 p.s.i. However, since the area of flange 58 is smaller than that of diaphragm 54, switch 69 will not open until p has fallen to a value below 96 p.s.i. Nevertheless, switch 69 will always open whenever p falls to a value (below 96 p.s.i.) such that the pressure differential 100-12 acting on flange 58 produces a force which exceeds the force of spring 74. Thus, if the area of flange 58 were, say one-fourth the area of diaphragm 54, switch 69 would always open whenever p fell to a value below (1004 4)=84 p.s.i., even when the rate of decrease of the pressure differential (p -p is less than 20 p.s.i.

While I have shown and described my invention as applied to a fuel supply system for aircraft engines, it is apparent that it may also be applied to any liquid flow control system, to indicate and function upon a change in pressure of the flowing liquid, when the rate of said change exceeds a selected predetermined rate.

I also desire it to be understood that while I have shown and described a preferred embodiment of my invention, I do not limit myself to the particular details of construction and arrangement of elements disclosed by way of illustration, as these may be changed and modified bythose skilled in the art without departing from the spirit of my invention or exceeding the scope of the appended claims.

I claim:

1. A liquid pressure rate of change sensing apparatus comprising: a first hermetically closed chamber containing an elastic fluid under a static control pressure, a second chamber containing liquid under varying pressure, first means, including a flexible diaphragm subject to the differential between said liquid pressure and said control pressure, for sensing a rate of decrease of said liquid pressure from a selected value, said means being so constructed and arranged as to be automatically actuated in response to said rate of decrease only when said liquid pressure decreases at a rate which exceeds a selected predetermined rate; and second means, actuated by said first means, for operating a remote control device, upon the occurrence of such rate of decrease in said liquid pressure.

2. An apparatus according to claim 1, including means for causing said second means to stop the operation of said device when said liquid pressure is restored to its original selected value.

3. An apparatus according to claim 1, wherein said device includes signal means for indicating at a remote point when said device is operated by said second means.

4. A liquid pressure rate of change sensing apparatus comprising: a first chamber containing liquid under a varying pressure whose rate of decrease is to be sensed, first means including a first flexible diaphragm subject to said liquid pressure, for sensing a rate of decrease of said pressure from a selected value; second means including a second hermetically closed chamber, for maintaining a static elastic fluid control pressure at a selected value; said first means being so constructed and arranged as to be responsive to the differential between said liquid pressure and said control pressure, and automatically actuated in response to said differential only when said sensed liquid pressure decreases at a rate which exceeds a selected, predetermined rate; and third means for operating a remote control device, upon the occurrence of such rate of decrease in said liquid pressure.

5. An apparatus according to claim 4, wherein the means for maintaining said static elastic fluid control pressure includes an air-tight chamber of selected maximum volumetric capacity and means for charging said chamher with a gas at said control pressure.

6. An apparatus according to claim 4, wherein said first means also includes a third chamber, separated from said first chamber by a second flexible diaphragm, and connected to a conduit containing said liquid; and means raw-m ze '11 eflfective upona decrease in the pressure of said liquid, actuated bythe movement of said second diaphragmyfor 1- causingsaidiliquid to escapefvrom both of said first causing the operationof a remote control device. .andthird ,chambers intosaid conduit at a different con- ;trolled rate of flow from each said chamber thereby R f n Cit d in th fil f thi patgnt creating a liquid pressure differential between said first [5 :and third chambers which acts on said second diaphragm UNITED STATES PATENTS with a force proportional to the difference in the rates 2,468,768 Malick May 3, 1-949 of liquid flow from each of first and third said chambers. 2,481,612 Nicholson Sept. 13, \1949 7. An apparatus according to claim ;6, having means 2,745,089 Levy .May 8, 1956 

